A network analyzer produces a measurement signal with a desired frequency on the basis of the frequency of an internal standard signal source. The frequency precision of such a measurement signal is no greater than the frequency precision of the internal standard signal. To satisfy the desire to utilize measurement signal frequencies that are more precise than the frequency of an internal standard signal source, a method is used in which an external standard signal with a more precise frequency than the internal standard signal is fed to the network analyzer and a highly precise measurement signal is obtained on the basis of this external standard signal.
In the implementation of the above described method, a voltage-controlled oscillator is used as the internal standard signal source, and the signal of the voltage-controlled oscillator is synchronized with the external standard signal by use of phase-locking technology. An example of this method is shown in FIG. 2.
In FIG. 2, 10 is a network analyzer and the structural elements of a network analyzer that are not related to this invention are omitted. An internal standard frequency signal source 19 is a voltage-controlled oscillator. In ordinary measurements, in which the frequency precision of internal standard frequency signal source 19 is sufficient for the accuracy of the measurement frequency, the input of internal standard frequency signal source 19 is held constant and a constant frequency, within the desired frequency precision, is output. Moreover, it is not necessary to connect an external standard frequency signal source 16.
A measurement frequency signal source 12 generates a measurement frequency signal of the desired frequency which is a rational-number multiple of the frequency of internal standard frequency signal source 19 by using a frequency synthesizing technology and feeds it to the other circuits in the network analyzer. A calculating and control part 13 calculates the factor for obtaining the desired measurement frequency and controls measurement frequency signal source 12.
The output of standard frequency signal source 19 is also connected to one input terminal of a phase comparator 17. The output of phase comparator 17 is connected to the input of internal standard frequency signal source 19 through an integrator 18 and thus a phase-locked loop circuit results. When the measurement is performed with the frequency precision of internal standard frequency signal source 19, as described above, the phase-locked loop is cut and the input of internal standard frequency signal source 19 is held at a constant value. FIG. 2 omits the circuit for switching the phase-locked loop on and off.
When one wants to make a measurement with a measurement frequency that is more precise than the frequency precision of internal standard frequency signal source 19, an external standard frequency signal source 16 is connected and the phase-locked loop is closed, forming the structure shown in FIG. 2.
Since the signal of external standard frequency signal source 16 is input into the other input of phase comparator 17, phase comparator 17 outputs a signal with a direct current component proportional to the phase difference of the signals of internal standard frequency signal source 19 and external standard frequency signal source 16. The direct current component of this output is returned to internal standard frequency signal source 19 through integrator 18 and controls the frequency of internal standard frequency signal source 19. Due to the negative feedback of this phase-locked loop, the frequency of internal standard frequency signal source 19 corresponds to the frequency of external standard frequency signal source 16.
In this manner, internal standard frequency signal source 19 generates a frequency with the precision of the frequency of external standard frequency signal source 16.
The phase-locked loop technology requires complex circuits containing analog circuits; it has many technological difficulties and increases cost. Furthermore, if phase noise is superimposed on the signal of the external standard frequency, this phase noise is also superimposed on measurement frequency signal source 12 through internal standard frequency signal source 19, and the phase noise of the measurement signal worsens. Under conditions in which the electromagnetic wave environment is a serious problem, such as in factories, phase noise is frequently superimposed on the external standard frequency signal, and measures such as shielding or filtering are ordinarily required to eliminate it.
As can thus be understood, if the frequency precision of an internal measurement frequency signal source in a prior art network analyzer was insufficient, the precision of the measurement frequency was improved by using a phase-locking circuit, on the basis of an external standard frequency signal source. This method was costly and used complex circuits. Further, when phase noise was superimposed on the external standard frequency signal source, complex measures were needed to remove it.
Accordingly, it is an object of this invention to provide a method for improving the frequency precision of a measurement signal with a simple circuit, and to reduce the cost thereof.